The present invention is related to the field of power semiconductor devices, and more particularly to circuits and structures for sensing thermal operating characteristics of power devices for device protection or other purposes.
Thermal protection of integrated power devices, such as integrated MOSFET power switches, is typically employed where a specific set of operating conditions and part mounting choices may cause the device to exceed its maximum operating temperature. One typical protection method involves measuring the temperature at the center of the power device and comparing it against a reference. When the measurement shows that the equivalent temperature has exceeded a predetermined value, such as 150° C., the device is shut down. A typical choice for measurement involves the use of a forward biased diode with a temperature coefficient of approx. −2 mV/° C. placed at the hottest part of the power device, typically its center. The diode voltage is compared with a predetermined threshold voltage corresponding to the absolute temperature of interest, such as 150° C. The output of the comparator is used to trigger shutdown or other protection operations as desired.
In protection schemes employing a separate sensing device such as a diode, typically the sensing device must have both electrical isolation and barrier guard rings for noise isolation from the power MOSFET. Interconnect must be routed to the center of the power MOSFET to bias the device and to carry the measurement voltage to the comparison circuitry. Such requirements typically result in a significant breakup of the center of the power MOSFET, thus increasing its silicon area.
There are also problems that arise due to the reliance upon measuring absolute temperature. First, the measurement is subject to variability arising from semiconductor process variations. Additionally, the technique is non-predictive, i.e., a power dissipating fault must continue until the die temperature is pumped up to an absolute value that trips the detection circuit. Testing of the protection circuitry may be very difficult or impossible, because testing risks destruction of the device. It would be desirable to overcome these shortcomings of prior device protection techniques.